Gene expression is regulated, in part, by the cellular processes involved in transcription. During transcription, a single-stranded RNA complementary to the DNA sequence to be transcribed is formed by the action of RNA polymerases. Transcription is initiated by the binding of RNA polymerase to characteristic recognition sequences in the promoter region of the gene. As used herein, the term “promoter” refers to the 5′ non-coding region of a eukaryotic gene that is involved in transcription initiation and regulation, which generally comprises between 100 and 1000 (or more) nucleotides upstream of the transcription start site. Generally, the promoter includes the transcription start site, and farther upstream from the transcription start site, the initiator region, comprising characteristic sequence motifs involved in binding RNA polymerase and initiating transcription, and cis-acting transcription control elements (also referred to as “promoter-proximal elements”) which extend several hundred bases upstream of the transcriptional start site and interact with trans-acting protein factors to regulate transcription. Such cis-acting control elements may be cell-or-tissue specific and may determine the responsiveness of transcription of the particular gene associated with the promoter to hormones and other endogenous signals. Other cis-acting control elements may affect the strength of the promoter (e.g., enhancers) or efficiency of transcription (e.g., the 5′ untranslated sequence downstream of the start site). Enhancers can occur upstream or downstream from the initiation site.
Tissue-specific promoters are particularly advantageous for use in transgenic modification of plants where spatial localization and/or developmental timing of gene expression is important, or where constitutive expression would be detrimental to the development and physiological function of the transgenically modified plant.
There is a continuing need for promoters that can be activated specifically in tissues involved in xylogenesis and primary and secondary xylem. Such promoters can be used to selectively modulate the expression of genes involved in secondary cell wall formation in plants, for example, by eliminating or reducing lignification (and increasing cellulose deposition) in secondary xylem, increasing the volume of particular secondary cell wall layers, and controlling the sites and levels of lignification and cellulose deposition.